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Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a high-performance, cell-oriented switching and multiplexing technology that utilizes fixed-length packets to carry different types of traffic. ATM is a technology that will enable carriers to capitalize on a number of revenue opportunities through multiple ATM classes of services; high-speed local-area network (LAN) interconnection; voice, video, and future multimedia applications in business markets in the short term; and in community and residential markets in the longer term.

In ATM networks, all information is formatted into fixed-length cells consisting of 48 bytes (8 bits per byte) of payload and 5 bytes of cell header (see Figure 2 ). The fixed cell size ensures that time-critical information such as voice or video is not adversely affected by long data frames or packets. The header is organized for efficient switching in high-speed hardware implementations and carries payload-type information, virtual-circuit identifiers, and header error check.

ATM is connection oriented. Organizing different streams of traffic in separate calls allows the user to specify the resources required and allows the network to allocate resources based on these needs. Multiplexing multiple streams of traffic on each physical facility (between the end user and the network or between network switches)—combined with the ability to send the streams to many different destinations—enables cost savings through a reduction in the number of interfaces and facilities required to construct a network.

ATM standards defined two types of ATM connections: virtual path connections (VPCs), which contain virtual channel connections (VCCs). A virtual channel connection (or virtual circuit) is the basic unit, which carries a single stream of cells, in order, from user to user. A collection of virtual circuits can be bundled together into a virtual path connection. A virtual path connection can be created from end-to-end across an ATM network. In this case, the ATM network does not route cells belonging to a particular virtual circuit. All cells belonging to a particular virtual path are routed the same way through the ATM network, thus resulting in faster recovery in case of major failures.

The basic principles of ATM as put forward by CCITT in Recommendation I.150 are:

  • ATM is considered as a specific packet oriented transfer mode based on fixed length cells. Each cell consists of an information field and a header, which is mainly used to determine the virtual channel and to perform the appropriate routing. Cell sequence integrity is preserved per virtual channel.

  • ATM is connection-oriented. The header values are assigned to each section of a connection for the complete duration of the connection. Signaling and user information are carried on separate virtual channels.

  • The information field of ATM cells is carried transparently through the network. No processing like error control is performed on it inside the network.

  • All services (voice, video, data, ) can be transported via ATM, including connectionless services. To accommodate various services an adaptation function is provided to fit information of all services into ATM cells and to provide service specific functions (e.g. clock recovery, cell loss recovery etc).

The benefits of ATM are the following:

  • high performance via hardware switching

  • dynamic bandwidth for bursty traffic

  • class-of-service support for multimedia

  • scalability in speed and network size

  • common LAN/WAN architecture

  • opportunities for simplification via VC architecture

  • international standards compliance

The high-level benefits delivered through ATM services deployed on ATM technology using international ATM standards can be summarized as follows:

  • high performance via hardware switching with terabit switches on the horizon

  • dynamic bandwidth for bursty traffic meeting application needs and delivering high utilization of networking resources; most applications are or can be viewed as inherently bursty; data applications are LAN–based and are very bursty, voice is bursty, as both parties are neither speaking at once nor all the time; video is bursty, as the amount of motion and required resolution varies over time

  • class-of-service support for multimedia traffic allowing applications with varying throughput and latency requirements to be met on a single network

  • scalability in speed and network size supporting link speeds of T1/bonded T1/E1 to OC–12 (622 Mbps) today and into the multi–Gbps range before the end of the decade; networks that scale to the size of the telephone network (i.e., as required for residential applications) are envisaged

  • common LAN/WAN architecture allowing ATM to be used consistently from one desktop to another; traditionally, LAN and WAN technologies have been very different, with implications for performance and interoperability

  • opportunities for simplification via switched VC architecture; this is particularly for LAN–based traffic that today is connectionless in nature; the simplification possible through ATM VCs could be in areas such as billing, traffic management, security, and configuration management

  • international standards compliance in central-office and customer-premises environments allowing for multivendor operation



ATM References:

ATM Forum

IEC: ATM Whitepaper

Cisco: ATM Internetworking

Cisco: ATM Switching

ATM Overview

The Cell Relay ATM FAQ

ATM Tutorial

ATM Tutorial

ATM News and Specifications

Discussion Paper on ATM Addressing



Very Good article sirI was not able to uderstand it but now i came to know the reason why the  length of the packet is fixed i.e 48 Bytes